The Natural Philosopher coughed up some electrons that declared:

Have you considered using a server to use your and one or more
neighbour's broadband lines so you both get twice the speed most of
the time?

Very complicated.

Not necessarily. It's called link aggregation or bonding and if you use an
ISP that supports it, and (for an easy life, but you could DIY it[1]) buy
their recommended router widget that does link aggregation/bonding then in
principle you could do this:

Fit master sockets with ADSL filters to both lines.

Bring both ADSL outputs into one house, and into the modem-router.

Send a bit of CAT5 back into neighbour's house.

If your usage times tend to randomly be different, then you'll tend to see
double normal speeds most of the time.

Now, both neighbours are already paying for the lines, so no extra cost
there.

http://www.aaisp.co.uk/kb-broadband-bonding.html

If the OP wanted to start a cooperative, the in principle, he could get
perhaps 3 or 4 neighbours together to share a common service, depending on
wire lengths between the houses.

Never tried it with ADSL, but I've done multiple gigabit link bonding on
linux servers and it works well.

Cheers

Tim

[1] If the ISP does it using 802.3ad

http://en.wikipedia.org/wiki/Link_aggregation

Then a little cheap Linksys WRT54S running OpenWRT could handle the customer
side quite nicely.